Field of the Invention
The present invention relates to a focus detecting apparatus which adopts an image-pickup-plane phase-difference detection method.
Description of the Related Art
One of phase difference detection methods reimages, as a pair of secondary images, optical images formed through different pupil regions of an image-pickup optical system, by a secondary imaging optical system on a focus detecting sensor, different from an image sensor, and detects a focus state of the image-pickup optical system based on a relative distance between the pair of secondary images. This phase difference detection method is referred to as the so-called TTL (Through The Taking Lens) secondary imaging phase difference detection method. This secondary imaging phase difference detection method divides a pupil of an image-pickup optical system by projecting a pair of line sensors (focus detecting sensors) by a field lens included in a secondary imaging optical system, onto the pupil.
Japanese Patent Laid-Open No. (“JP”) 62-133409 proposes a focus detecting apparatus which adopts this secondary imaging phase difference detection method and changes a calculation range for a correlation calculation based on defocus information of a lens in detecting a phase difference using the correlation calculation. The defocus information means a defocus amount from a current focus position to the infinity end and a defocus amount from the current focus position to the short distance end. An infinity-side shift amount is calculated based on a defocus amount from a current focus position to the infinity end, and on the K and G values. Similarly, a short-distance-side shift amount is calculated based on the defocus amount and on the K and G values. This method eliminates a wasteful calculation time by performing the correlation calculation in the defocus range detectable by the mounted lens.
Another phase difference detection method is the so-called image-pickup-plane phase-difference detection method which provides a pair of light-receiving elements for each of two-dimensionally arrayed micro lenses in an image sensor and divides a pupil of an image-pickup optical system by projecting the pair of light receiving elements onto the pupil.
In the image-pickup-plane phase-difference detection method, as the defocus amount increases, a detected defocus amount becomes larger than the actual value due to the image signal deterioration in addition to a phase shift. Therefore, when a shift range for the calculation is set based on the defocus amount of a lens similar to JP 62-133409, a shift position representative of an object position may be located outside the shift range, and a shift position representative of another object position within the shift range may be detected. In addition, a lens, which is to be driven based on its target driving amount calculated from a detected shift amount, may be driven by a defocus amount larger than the lens drivable amount.